Thermocouple circuit



April 1962 R. R. BOCKEMUEHL ET AL 3,030,575

THERMOCOUPLE CIRCUIT Original Filed Jan. 11, 1954 f a z??? z/ we WK BY25;? 12522212 ATTORNEY United States Patent 3,030,575 THERMOCOUPLECIRCUIT Robert R. Bockemuehl, Birmingham, and Robert J. Moiiat, OakPark, Mich., assignors to General Motors Corporation, Detroit, Mich., acorporation of Delaware Original application Jan. 11, 1954, Ser. No.403,398. Divided and this application Sept. 29, 1958, Ser. No. 764,034

3 Claims. (Cl. 324-51) This application is a division of applicationS.N. 403,398 filed January 11, 1954 (now abandoned).

Our invention relates to controls for gas turbine engines and othercombustion devices or thermal plants. The preferred embodiment of theinvention in a safety control for a gas turbine engine with twocombustion chambers is described herein.

In brief, the control described herein employs two temperatureresponsive devices such as thermocouples, one in each combustionchamber, the responsive devices being connected in opposition andoperative through suitable relay mechanism to shut off fuel to theengine when significant temperature unbalance occurs.

The invention which is the subject of this application lies particularlyin a thermocouple circuit including means for checking the continuity ofthe circuit to assure that it is in condition to perform its function.

The principles and advantages of the invention will be apparent to thoseskilled in the art from the succeeding detailed description and theaccompanying drawing which is a schematic diagram of a gas turbine fuelsystem embodying the invention.

The fuel system is illustrated in combination with a gas turbine enginewhich comprises a compressor supplying combustion chambers 11 and 12,and a turbine wheel 13 driven by combustion products from the chambersand driving the compressor through a shaft 14. This part of the engineserves as a gas generator for a power turbine wheel 16 driving a shaft17. Fuel nozzles 18 in the combustion chambers are supplied from a fueltank 19 through pump 21, fuel control 22, a normally closedsolenoid-opened valve 23, and line 24.

Thermocouples 31 and 32, which may be of a known quick-responsehigh-temperature type, are mounted adjacent the discharge ends ofcombustion chambers 11 and 12. These couples are in reverse seriesconnection so that the thermoelectric E.M.F.s of the two are opposed,and the resultant is substantially zero when the two are at the sametemperature. The series circuit may be traced through thermocouple 31,lead 33, switch 36, coil 37 of a sensitive relay 35, resistor 38, lead39, thermocouple 32, and lead 41. Relay 35 is of a known galvanometrictype. Coil 37 operates an armature 44 in either direction in response todirection of current in the coil. A contact arm 46 actuated by thearmature will engage either of two fixed contacts 47 in response toexcitation of coil 37 above a predetermined value. The contact arm ismagnetically urged into, and held in, engagement with the contacts untilreleased by energization of a reset coil 48.

Closure of contacts 46, 47 energizes solenoid valve 23 to close the fuelline by means of the following circuit: A battery 51 is connected toground, and through switch 52, line 53, relay coil 54, switch 56, andcontacts 47 (when closed) to ground. The battery also is connectedthrough line 53, switch 58, back contacts 59 which may be opened by coil54, and solenoid 61 of valve 23 to ground. Relay coil 54 is shunted by aresistor 63 and a signal light Patented Apr. 17, 1962 64 which isenergized when contacts 47 close. The reset coil 48 is energized fromthe battery through line 53, resistor 66, and normally open push buttonswitch 67.

In normal operation of the electrical system, switches 52, 36, and 56are closed. Switch 58 is closed to energize valve solenoid 61 to admitfuel to the engine upon starting. Contacts 47 are normally open andcontacts 59 normally closed. If, however, the differential of thethermocouples 31 and 32 exceeds the threshold of relay 35, contacts 47close, energizing warning light 64 and relay coil 54, opening contacts59 and cutting off the fuel at valve 23. Because of the inherent thermallag of thermocouples, the response of the relay is a function of boththe magnitude and duration of the temperature dif ference between thecombustion chambers. J

Light 64 is energized to indicate that the dilferential temperaturesafety circuit has cut out the engine. It re mains cut out until switch67 is closed to reset relay 35. Switch 56 may be opened to disable theenergizing circuit of relay 54, if desired. Thus, switch 56 may heclosed only when starting the engine if the system is desired to beoperative only then. Switch 52 is an overall energizing switch for thesystem. v

A test circuit for the thermocouple circuit is provided, Energized line53 is connected through high resistance 71 to the circuit on one side ofswitch 36 while the circuit is grounded through high resistance 72 onthe other side of switch 36. With switch 36 closed, the small currentbled through resistors 71 and 72 in series passes through this switchand does not aiiect the relay 35. To test eon tinuity of thethermocouple circuit, switch 36 i opened, whereupon the battery 51 isconnected through resistor 71, relay coil 37, the thermocouples, andresistor 72 to ground. If the circuit is complete, relay 35 operates, energizing lamp 64.

It will be apparent that the invention may be employed in otherenvironments than that described and may be embodied in diversecombinations of apparatus within the Y scope of the invention.

We claim:

1. In combination, a control circuit comprising at least onethermocouple, a galvanometer, and a normally closed switch in series,and means for testing continuity of the circuit comprising a low-currentsupply means connected to opposite ends of the switch so that thegalvanometer is energized by the said source through the thermocouplewhen the switch is opened.

2. A control circuit comprising, in combination, thermocouple means,current-responsive means, and a normally closed switch connected inseries, and means for testing the continuity of the series circuitcomprising a limited current supply means connected to the seriescircuit at 0pposite sides of the switch so that the current-responsivemeans is energized by the source through the thermocouple means when theswitch is opened and the series circuit is continuous.

3. A circuit as recited in claim 2 in which the thermocouple meanscomprises two thermocouples in series opposition.

Podell Dec. 18, 1956

